1. Field of the Invention
The invention relates to an electroacoustic transducer comprising a diaphragm which is substantially centro-symmetrical with respect to a transducer axis and which is capable of vibrating in the direction of the transducer axis, and comprising a voice coil connected to the diaphragm in the area of the rear volume of the transducer, and comprising a magnet system which is substantially centro-symmetrical with respect to the transducer axis, this magnet system being accommodated in a housing of the transducer and comprises a magnet, a cover disc adjoining the magnet, and a substantially pot-shaped yoke comprising a core disc, which also adjoins the magnet, and a hollow cylindrical yoke portion which projects from the core disc in a direction parallel to the transducer axis, and which forms an annular air gap between the cover disc and the hollow cylindrical yoke portion, the voice coil being arranged in the annular air gap, and comprising a substantially annular closing member adapted to close the rear volume, which closing member having a contact surface which extends parallel to a circle which is concentric with the transducer axis, this contact surface facing a contact portion of a part of the transducer, this part also bounding the rear volume, and the closing member having at least one passage which terminates in its contact surface, for realizing an acoustic friction between the rear volume and the acoustic free space surrounding the transducer.
2. Description of the Related Art
An electroacoustic transducer of the type defined in the opening paragraph is commercially available from Philips Electronics under the type designation WD 00917, and is known therefrom. The known transducer comprises a substantially cylindrical magnet system which includes a circular cover disc, a round disc-shaped magnet and a pot-shaped yoke which comprises a round disc-shaped base from which a hollow cylindrical yoke portion projects at the location of its perimeter, this portion changing into peripheral pot wall which extends transversely to the transducer axis. In the known transducer, this magnet system is accommodated in a plastic transducer housing. In the known transducer, the closing member for closing the rear volume of the known transducer is formed by a ring-like closing cap, which is also made of a plastic and which comprises a hollow cylindrical closing wall, which extends parallel to the transducer axis, and an annular closing disc, which is connected to this closing wall and which has a circular opening. The ring-like closing cap provided as the closing member is connected to the transducer housing with its hollow cylindrical closing wall, i.e., with the peripheral face of this wall, by ultrasonic welding in a mechanically rigid and acoustically sealed manner. With its opening, the annular closing disc is slid onto the pot-shaped yoke of the magnet system, i.e. onto the hollow cylindrical yoke portion, thus forming a contact surface of the closing cap, this contact surface being formed by the bounding surface of the opening, being concentric with and extending parallel to the transducer axis of the known transducer, and engaging with a contact portion of the hollow cylindrical yoke portion of the yoke of the magnet system, this contact portion also extending parallel to the transducer axis. In the known transducer, it is thus necessary to connect the closing member, i.e., the closing cap, to the transducer housing in a mechanically rigid as well as acoustically sealed manner and, in addition, to adapt the inner diameter of the annular closing disc of the closing cap and the outer diameter of the hollow cylindrical yoke portion of the yoke system accurately to one another. Both the acoustically sealed connection to the housing of the transducer and said adaptation of the diameters constitute an additional operation, which makes the transducer more expensive and should therefore be avoided.